SQ1: Foraging Theory and Encounter Rates
Consider any stimulus (heat, chemicals, light, sound, other mechanical displacement, gravity, electricity, or magnetism). Describe how it might be used to increase encounter rate with food in any marine environment by any kind of organism.
Outline:
Answer:
Zooplankton in the Northeast Pacific can dramatically increase the rate at which they encounter prey by migrating -- in response to an unknown stimulus -- approximately 1200 meters from mid-water depths to the top of hydrothermal plumes emanating from the Juan de Fuca spreading center.
Prey encounter rates for an individual copepod can be assumed to depend not only on the radial component of the predator's velocity relative to its quarry, but also on the local concentration of prey and the encounter radius. In the open Pacific at mid-water depths, the role of stimuli in affecting encounter rates may be most important in increasing the effective radius of encounter and the directionality of the relative velocity. Rather than swimming up in the water column and drifting down (Jumars, 1/16/97) randomly, a functional response of a copepod with the ability to detect vibrations or spectral characteristics of nearby prey would be to make slight adjustments to its swimming and sinking trajectories, reaping the benefit of a small bias.
Greater increases in encounter rate are possible, however, if zooplankters respond appropriately to stimuli which lead to dramatic increases in prey concentrations. High abundances of zooplankton, including individuals from mid-water faunal assemblages have been sampled in a narrow depth range (~100 meters) above, and not within the Endeavour Segment hydrothermal effluent (Burd et al, 1992). In analogy to the response of an organism in the benthic boundary layer to an attractant, a copepod may swim downwards when stimulated by a signal (the lipid-rich (buoyant) eggs, perhaps?) from a deep, chemosynthetic ecosystem. Such a vertical migration leads to the top of the hydrothermal plume (assuming that plume and copepod experience similar advection), and more bountiful prey. The abundance of appropriately- sized protozoans feeding upon chemosynthetic microbes will increase, and bacteria upon appropriately sized mineral precipitate particles may be encountered (Baross, personal communication).
Burd et al completed discrete plankton tows "suggesting that zooplankters approach the plume closely, but cannot tolerate conditions within the core, possibly because of the toxicity of chemicals and metals within the plume." This observation reveals that zooplankters enjoy an additional opportunity to increase their encounter rates: the epiplume distribution of prey and predators is likely planar in contrast with the 3-dimensional, patchy prey distribution of mid-water environments. Additionally, prey should be strongly zonated, perpendicular to strong gradients in light transmission, chemical constituents, temperature, and salinity.
This distribution of high concentration prey alone would improve the encounter rate for mid-water copepods, but if the search behavior were also altered -- to one in which predator motion was primarily perpendicular to the plume gradients -- huge increases in encounter rate would be realized! What stimuli could guide such a functional response? Turbulent fluid motions on the plume boundary at scales large relative to the copepod would prohibit the zooplankters' use of the gradients themselves, but utilization of a mid-water stimulus response (perhaps to prey vibrations) could be effective in boosting the radial component of relative velocity. Such a strategy would be crucial, too, in moving against the mean deep water advection through large-scale gradients in prey concentration -- a behavior requisite to maintaining the advantageous position over the chemosynthetic ecosystems.
References:
Baross, J. Personal communication 1/15/97